In recent years, studies on T cell recognition of protein antigens have focused on using small peptides to identify immunodominant regions of the protein. These studies have specifically avoided the analysis of structural changes, in the native molecule outside of the antigenic determinant, that may influence T cell recognition. However, the apparent specificity of an immunodominant region may substantially depend on neighboring amino acids that are not directly recognized by the T cells. In this proposal, it is hypothesized that specific amino acid alterations outside the immunodominant region will have an affect on T cell recognition. The Staphylococcal nuclase enzyme, a simple 149 amino acid protein, will be used to study T cell recognition. The immunodominant region of this protein has been identified for H- 2a-restricted T cell clones. Moreover, mutant proteins are available with single amino acid changes that span the length of the molecule. These mutant proteins provide a unique opportunity for studying how structural changes in the molecule influence Staph-nuclease-specific T cell recognition. Specifically, this proposal will determine if these changes have an affect on antigen recognition by either altering that portion of the antigen that is recognized by the T cell receptor or by the Ia molecule.